Download Power Point for Chapter 3, Lesson 1

Genes and Chromosomes
Chromosomes are
found in the
nucleus of a cell.
Write these down p. 74
Chromosome: a
long chain of genes
Chapter 3 Section 1
What is Heredity?




Austrian monk born in 1822
Son of a farmer
Ordained as a priest
Lived in a monastery where
over many years, he
performed hundreds of
experiments on pea plants
relating to heredity
 Known as the FATHER OF
GENETICS
Heredity
Passing of
physical
characteristics
from parent to
offspring
Mendel’s study of pea plants
would forever change biology
p. 74
Specific
characteristics,
such as stem
height or seed
color
Remember…
Allele: Different forms of
Write this…
a gene
Mendel’s Experiments
p. 75
 Genetics: The
scientific study
of heredity
• Mendel wondered why different pea
plants had different characteristics:
some tall, some short, some with
green seeds, some with yellow seeds…
MENDEL’S EXPERIMENTS
 Mendel “crossed” pea plants with
different traits and recorded results
 “Crossed” means he took pollen from
one plant and put it on another plant to
fertilize it
 FERTILIZATION: when egg and
sperm join together
Just listen…no need to
highlight
Before we learn about Mendel’s
experiments, let’s review the basics of
sexual reproduction in flowering plants…
Flowers have both male and female reproductive structures.
• The female part of the
flower: pistil
• The pistil produces egg
cells.
p. 75
• The male part of the
flower: stamen
• The stamen produces
pollen which contains
sperm cells.
The birds and the bees of plants…
When pollen lands on the pistil of a flower, sperm
travels down to the egg cell, and the result is
fertilization
Fertilization
produces a tiny
embryo, which
is enclosed
inside a seed
Pea plants are usually self pollinating
Since the reproductive structures are inside the
flower, the sperm of the flower will usually
fertilize the egg of the same flower.
Resulting
embryos will
Even though have the same
characteristics
sexual
reproduction as their parents
has occurred –
there is only 1
parent – 2 sex
cells though…
MENDEL’S EXPERIMENTS
Crossing Pea Plants
Mendel devised a way to cross-pollinate pea plants.
Mendel started with PUREBRED
ORGANISMS
 PUREBRED: The offspring of
many generations that have the
same form of a trait
 Example: Purebred tall pea plants
always come from tall parent plants.
Before we move on, you
must be familiar with the
following terms:
P generation: Parent generation
F1 generation: First generation of
offspring
F2 generation: second generation
of offspring
Which
thumb do
you put on
top?
Hybrids: offspring of parents with
different traits
F1 Offspring
p. 76
 The offspring from the parent
cross
 Filial comes from filia and filius – the Latin
words for “daughter” and “son”
F1 Offspring
 The offspring from the parent
cross – (1st generation offspring)
 Mendel crossed purebred tall plants with
purebred short and all F1 offspring were tall.
F2 Offspring
 The offspring from the F1
generation (2nd generation offspring)
 Mendel let the hybrid tall plants from the first
generation self pollinate – the result was a mix
of tall and short plants. ¾ were tall and ¼
were short.
• Evidently the F1 “tall” offspring must have been
carrying the short trait, but it had been hidden.
• The short trait had been passed down to the offspring
and it reappeared in the F2 generation.
Experiments with other traits
 Mendel studied other traits such as seed shape
and color, pea pod shape and color.
 In all of Mendel’s crosses, he found that
only one form of the trait appeared in the
F1 generation. However in the F2
generation, the “lost” form of the trait
always reappeared in about ¼ of the
plants.



Example…P generation (tall x short)
F1 generation = all tall
F2 generation = ¾ tall, ¼ short
Results of a Cross –
figure 2 pg. 76
Results of a Cross –
figure 2 pg. 76
How Do Alleles Affect Inheritance?
 Mendel reached several conclusions
P. 77
 Sets of genetic information must
control the inheritance of traits
 The factors that control each trait
exist in pairs
 The female parent contributes one
factor, the male contributes the
other
 One factor in a pair can mask, or
hide, the other factor
 When fertilization occurs, the new
offspring will have two alleles for
every trait
How Do Alleles Affect Inheritance?
Gene
 Factors that control a trait
 A segment of DNA
Do you know this Gene???
ALLELES
Different forms of a gene
EXAMPLE: The gene that controls stem
height in peas has one allele for tall
stems and one for short stems.
ALLELES ARE INHERITED
Our little sweet
pea!!
 Each pea plant inherits two
alleles from its parents – one
allele from MOM (the egg)
and the other from DAD (the
sperm)
ALLELES ARE INHERITED
 An organism’s traits are controlled by
the alleles it inherits from its
parents. Some alleles are dominant,
while other alleles are recessive
 A dominant allele can cover up or
mask a recessive allele
Dominant vs. Recessive Alleles
 Dominant alleles: trait that
always shows up when the allele
is present in the DNA
 Recessive Alleles: trait that is
“hidden” whenever the dominant
allele is present in the DNA

(this trait will only show up if the dominant allele is not
present)
Circle the picture of each dominant
form of the trait in the P generation.
Predict: Under what conditions
would the recessive form of one
of these traits reappear?
Predict: Under what conditions
would the recessive form of one
of these traits reappear?
If an offspring
has two
recessive
alleles.
Alleles in Mendel’s Crosses
 Hybrid: An organism with two
different alleles for a trait
 (1 dominant and 1 recessive)
 The F1 plants were all hybrids - one tall allele
and one short allele
 The dominant allele masked the recessive
allele, so all the plants were tall in the F1
Symbols for Alleles
 Geneticists use letters to
represent alleles
 Dominant allele – represented by a
capital letter
 Recessive allele – represented by a
lowercase version of the same letter
 Example: T = tall stems
t = short stems
TT = tall plant
Tt = tall plant
Tt = short plant
Dominant and Recessive Alleles
Figure 4
•
•
•
T
Fill in the missing
allele symbols and
descriptions.
Use the word bank
to complete the
statements.
(Terms will be
used more than
once).
Draw the two
possible ways the
F2 offspring could
look.
t
Tall
t
recessive
dominant
Hybrid
dominant
dominant
recessive
recessive
Apply it!
WW and ww
All will have long wings; Ww
P. 79
About ¾ will have long
wings and ¼ will have
short wings
Significance of Mendel’s Contribution
 Mendel’s discovery of genes and
alleles eventually changed
scientists’ minds about heredity
 Before Mendel, most people thought traits were
just a blend of the parent’s characteristics.
 Mendel showed that offspring traits are
determined by the individual, separate alleles
inherited from each parent.
Significance of Mendel’s Contribution
 Known as the Father of Genetics
Assess Your Understanding:
Look at questions on page 79
 2 a. The dominant allele for tall
stems masks the recessive allele for
short stems
 2 b. No!!! A Hybrid plant has one
dominant and one recessive allele, so
it must be tall. A short plant can only
have recessive alleles.